The present invention related generally to optical circuits, and, more specifically, to an optical circuit in which fibers interconnect input and output ports.
The use of optical fiber circuits to manage the fibers between optical connections has increased dramatically in recent years. The primary reason for this dramatic increase is the constant desire to increase the density of optical connections. In other words, there is a strong impetus to increase the number of fibers that can be optically connected in a given space. As the density of the optical connections increases, the need to manage the ganglia of fibers associated therewith increases in kind. As used herein, the term xe2x80x9coptical fiber circuitxe2x80x9d refers broadly to an arrangement of fibers that interconnects one set of ports with another set of ports. The arrangement of fibers is fixed to some medium to prevent its movement. The terms xe2x80x9cinput portxe2x80x9d and xe2x80x9coutput portxe2x80x9d are used herein for illustrative purposes to provide a distinction between the two sets of ports. It should be understood, however, that this designation should not limit the invention to a particular propagation of light through the ports.
The particular use of optical fiber circuits varies although a preferred use is in an optical cross-connect. As used herein, the term xe2x80x9coptical cross connectxe2x80x9d refers generally to any device that optically interconnects groupings or xe2x80x9cnodesxe2x80x9d of fibers with other nodes of fibers. Typically, but not necessarily, a node corresponds to a port on the optical circuit. The term xe2x80x9cperfect shufflexe2x80x9d as used herein refers to a particular configuration of an optic cross-connect in which each output node contains a fiber from each input node. Thus, in a perfect shuffle, the number of interconnections for each node is equivalent to the number of nodes. For example, to effect the interconnection of eight input nodes to eight output nodes, there must be eight interconnections per node.
The applicants have found that as the number of input and output ports (or nodes) increases in an optical fiber circuit, the problems facing the fiber layout increase as well. Of particular concern is the introduction of skew. Skew refers to a differential in length among the fibers connecting the various ports together. If this differential becomes large, it can result in a significant difference in the port-to-port transmission time among the various ports. This is generally unacceptable. The applicants have determined that running the individual fibers along straight lines between ports is an effective approach to reducing skew. Such an approach minimizes the port-to-port distances compared to other fiber layout approaches in which the fibers are channeled together in a common bus from one set of ports to the other.
The direct port-to-port approach results in fibers traversing diagonally across the optical circuit since the fibers of a given input port are interconnected to each of the output ports. This is not problematic in itself; however, when the number of ports increases, the incidence of xe2x80x9cfiber stackingxe2x80x9d among the diagonals increases exponentially. More specifically, a symmetrical arrangement of fibers that interconnects input and output ports along straight lines results in the stacking of multiple fibers at certain points on the optical circuit. The number of fibers in a stack can be quite high, especially as the number of ports and fibers increases.
Applicants have identified a number of problems associated with the stacking of multiple fibers. In particular, the fibers toward the top of the stack are forced to bend more to get over the stack. Once the stack is about three of four fibers high, the amount of bending experienced by subsequently stacked fibers to clear the stack may exceed the fiber""s minimum bend radius. This introduces optical losses and can compromise the integrity of the optical transmission path in general.
Aside from degrading optical performance, fiber stacking also introduces structural problems. Specifically, the height of the stack itself becomes an issue since, as a high spot, it tends to be subjected to external aggression more so than the lower lying portions of the optical circuit. After repeated scrapping and knocking, the top fibers"" performance can suffer. A tall stack also presents problems in laminating the fibers since the stack serves to separate the top and bottom layers of the optical circuit, thereby making adhesion between them more difficult.
Therefore, there is a need for an optical circuit design which provides for fiber shuffling between the input and output ports but which avoids the aforementioned problems of fiber stacking. The present invention fulfills this need among others.
The present invention provides for an optical circuit that minimizes skew but avoids fiber stacking by using an asymmetrical fiber arrangement for interconnecting the input and output nodes. The applicant has found that by asymmetrically arranging the fibers, the incidence of fiber stacking is reduced considerably. Furthermore, applicants have found that a sufficient degree of asymmetry may be introduced into the circuit if the point at which the fibers converge/diverge at the various ports is staggered.
Accordingly, one aspect of the invention is an optical circuit in which the fibers interconnecting the ports are arranged asymmetrically about at least one axis of the fiber circuit. In a preferred embodiment, the optical circuit comprises: (a) a body having an x and y axis and two sides substantially parallel to the y-axis and separated along the x-axis; (b) a plurality of first ports along one of the two sides, each first port comprising a number of substantially paralleled fibers; (c) a plurality of second ports along the other of the two sides, each second port comprising a number of substantially parallel fibers. The ports are interconnected with the fibers such that (i) the fibers of a given second port comprise a fiber from each of the first ports, and (ii) the fibers interconnecting the first and second ports are arranged asymmetrically about at least one of the x or y axes. Preferably, the fibers of a given port diverge from parallel on the body at a divergence point and the divergence points of at least one of the first ports or the second ports are staggered along the x axis. Furthermore, the fibers interconnecting the first and second ports preferably are arranged symmetrically about the y axis.